1. Field of the Invention
This invention relates to a focus detecting apparatus for detecting the focus adjusted state of an object image formed by an objective, lens and to a camera containing such apparatus therein and to which the objective lens is removably mountable or secured.
This invention further relates to a focus detecting apparatus provided with defocus amount detecting means for repetitively detecting the defocus amounts of a plurality of object areas, operating means for starting the defocus amount detecting operation of said defocus amount detecting means, and selecting means for selecting at least one of said plurality of object areas.
2. Related Background Art
As a focus detecting apparatus in a camera, there is well known a method whereby light beams from an object area passed through different exit pupil areas of a photo-taking lens are imaged on a pair of line sensors and the amount of displacement of the relative position of a pair of image signals obtained by photoelectrically converting the object image is found to thereby detect the defocus amount of the object area.
This method uses one set of focus detecting systems (optical systems and sensors) and therefore can detect only the defocus amount of one object area, but there have been proposed a number of methods of detecting the defocus amounts of a plurality of object areas by preparing a plurality of sets of detecting systems.
In the latter method, there are a plurality of object areas and therefore, there are a plurality of defocus amounts detected. However, the object area on which it is desired to focus the camera is one or two at greatest (in this case, for example, the information intermediate of the two is used for focus adjustment) and therefore, it is necessary to select an object area under some judgement condition and effect the focusing of the photo-taking lens in accordance with a defocus amount corresponding to the selected area.
A popular selection method is a method of selecting the object area closest to the camera.
However, in the selection method as described above, the object area is reselected each time focus detection is repetitively effected and therefore, there occurs the following inconvenience when a photo-taking lens of the rear focus type (in which the angle of field varies appreciably when focusing is effected) is mounted on the camera.
Let it be assumed, for example, as a result of the focus detection at a certain focus position, an object in a marginal area is selected and focus adjustment is effected on the basis of the defocus amount of that area. In the lens of the rear focus type, the image in the marginal field of view varies greatly when the focus is changed and therefore, when the process is repeated in the second focus detection at a new focus position, another object area is now selected and focus adjustment is effected on the basis of the defocus amount of that area, and in some cases, oscillation in the focus adjusting operation of the photo-taking lens may result.
In the selection method as previously described, the selected area is not changed even when the object varies and the object area being selected is no longer the closest object. Conversely, if an attempt is made not to hold the selected area, the focus adjusting operation of the photo-taking lens will become oscillated as previously mentioned, and this is very inconvenient. Or there is the inconvenience wherein focus detection will become impossible if the object disappears from the object area being selected or an object having no contrast comes into the object area being selected while focus detection is repetitively effected.